The present invention relates generally to Space Based Augmentation Systems (SBAS) and methods to improve the accuracy, availability, and integrity of basic Global Positioning System (GPS) service. In particular, the present invention is directed to SBAS and methods using ionospheric bounding data to determine a geographical correction source.
The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of satellites and corresponding ground stations. Currently, approximately twenty-four satellites are used in the GPS. Each satellite continually broadcasts its location in space along with the current time from an internal clock. GPS receivers are able to determine their position by receiving and analyzing signals transmitted from the satellites. Two-dimensional locations are able to be determined by analyzing signals from three satellites, and three-dimensional locations are able to be determined by analyzing signals from four or more satellites. A GPS receiver determines its location by determining its distance from the GPS satellites based on the received signals and then performing a geometric triangulation on these distance measurements. GPS will be described in more detail below.
Although the current GPS has been successful, it has several shortcomings that affect the accuracy of positioning calculations. For example, GPS satellite signals are subject to errors caused by ionospheric disturbances and satellite orbit discrepancies. Ionospheric and tropospheric refraction can slow satellite signals and cause carriers and codes to diverge. Because ionospheric disturbances vary greatly from location to location, these errors are difficult to correct with civilian-type GPS receivers. These and other errors are described in more detail below.
Differential GPS (DGPS) can improve the accuracy of position measurements. DGPS uses an extra stationary receiver at a known location as a reference point. The stationary receiver measures GPS signal error by comparing its exact, known location with the location derived from the GPS signals. The reference receiver sends timing error measurements to mobile GPS receivers that allow these GPS receivers to correct for errors and get a more accurate position measurement. DGPS assumes that the reference point and other receivers will encounter similar errors. One example of DGPS is the Radio Technical Commission for Maritime (RTCM) Services, provided by the U.S. Coast Guard, which provide DGPS correction signals.
Space Based Augmentation Systems (SBAS) have been developed to further account for the above-described errors and better improve the accuracy, availability and integrity of the GPS. Wide Area Augmentation System (WAAS) is one type of Space Based Augmentation System (SBAS) used in North America. The Federal Aviation Administration (FAA) developed and uses WAAS to aid in landing aircraft. The FAA is developing a Local Area Augmentation System (LAAS) with reference receivers located near runways to further aid in landing aircraft, particularly in zero visibility conditions. One benefit of WAAS is that it provides extended coverage both inland and offshore compared to a land-based DGPS. Another benefit of WAAS is that it does not require additional DGPS receiving equipment.
Other governments are developing SBAS. In Asia, the SBAS is referred to as the Japanese Multi-Functional Satellite Augmentation System (MSAS). In Europe, the SBAS is referred to as the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other SBAS systems.
As will be described in more detail below, the WAAS is based on a network of wide area ground reference stations (WRSs) that are linked to cover a service area including the entire U.S. and some areas of Canada and Mexico. The number of WRSs is currently about twenty-five. The WRSs are precisely surveyed so that the exact location of each WRS is known. Signals from GPS satellites are received and analyzed by the WRSs to determine errors in the signals, including errors caused by the ionospheric disturbances described above. Each WRS in the network relays its data to a wide area master station (WMS) where correction information is computed. The WMS calculates correction messages for each GPS satellite based on correction algorithms and assesses the overall integrity of the system. The correction messages are then uplinked to Geostationary Communication Satellites (GEOs), also referred to herein as SBAS satellites or more particularly as WAAS satellites, via a ground uplink system (GUS). The SBAS satellites broadcast the messages to GPS receivers within the coverage area of the SBAS satellites on the same frequency as the GPS signals (i.e., L1, 1575.42 MHz). GPS receivers with SBAS capabilities are capable of using the correction messages to correct for GPS satellite signal errors caused by ionospheric disturbances and other inaccuracies. The SBAS satellites also act as additional navigation satellites for the GPS receivers, thus, providing additional navigation signals for position determination.
With respect to WAAS, the correction messages currently are uplinked to two WAAS satellites. The GPS receiver is capable of being positioned within the coverage area of both of these WAAS satellites such that the receiver is capable of receiving WAAS correction messages from either of these WAAS satellites. Additional GEOs are capable of being used for a more comprehensive SBAS that provides a larger coverage and more redundancy. As such, a GPS receiver is capable of being positioned within the coverage area of two or more of these SBAS satellites such that the receiver is capable of receiving SBAS correction messages from any one of these SBAS satellites.
The accuracy, desirability and/or equivalency of SBAS correction messages are not necessarily the same for the various SBAS correction sources, e.g. SBAS satellites. Accordingly, there exists a need for an improved method and system for determining the appropriate or desired geographical correction source for SBAS corrections. There is further a need for improved systems and methods which benefit from the SBAS data while utilizing a minimal amount of memory and processor resources, particularly in the GPS device.
The above mentioned problems with the accuracy, availability and integrity of GPS service, as well as other concerns for system resources, are addressed by the present invention and will be understood by reading and studying the following specification. SBAS and methods are provided for improving the accuracy, availability and integrity of GPS service. Specifically, the present invention provides a GPS device with the correction messages which are from the most accurate and desirable source while at the same time conserving processor and memory resources of the device.
In one embodiment of the present invention, a method for determining a corrections source in a space based augmentation system (SBAS) for use by a global positioning system (GPS) device is provided. The method includes receiving GPS data and receiving an SBAS signal from a first correction source. The SBAS signal contains a number of ionospheric mask messages. Each ionospheric mask message includes a number of ionospheric mask bits with each bit representing a single grid point in a single ionospheric band. The ionospheric mask messages are analyzed and an abbreviated bounding region around a group of similar type grid points is constructed.
In another embodiment of the present invention, a method for determining a corrections source in a space based augmentation system (SBAS) is provided. The method includes receiving ionospheric mask messages from a first GEO satellite. The ionospheric mask messages are dynamically analyzed to determine a first abbreviated bounding region for a group of ionospheric grid points. A position of a GPS receiver device is determined. The method further includes determining whether the position of the GPS receiver device is within the first abbreviated bounding region.